Exploring the potential applications of lead-free organic–inorganic perovskite type [NH3(CH2)nNH3]MCl4 (n = 2, 3, 4, 5, and 6; M = Mn, Co, Cu, Zn, and Cd) crystals

The organic–inorganic hybrid perovskite compounds have been extensively studied since the dawn of a new era in the field of photovoltaic applications. Up to now, perovskites have proven to be the most promising in terms of power conversion efficiency; however, their main disadvantages for use in solar cells are toxicity and chemical instability. Therefore, it is essential to develop a hybrid perovskite that can be replaced with lead-free materials. This review focuses on the possibility of applying lead-free organic–inorganic perovskite types [NH3(CH2)nNH3]MCl4 (n = 2, 3, 4, 5, and 6; M = Mn, Co, Cu, Zn, and Cd) crystals. We are seeking organic–inorganic hybrid perovskite materials with very high temperature stability or without phase transition temperature, and thermal stability. Thus, by considering the characteristics according to the methylene lengths and the various transition metals, we aim to identify improved materials meeting the criteria mentioned above. Consequently, the physicochemical properties of organic–inorganic hybrid perovskite [NH3(CH2)nNH3]MCl4 regarding the effects of various transition metal ions of the anion and the methylene lengths of the cation are expected to promote the development and application of lead-free hybrid perovskite solar cells.

perovskite based on methylammonium tin iodide was developed and showed initial efficiencies of up to 6.4%.And, the band gap energy relevant for photovoltaic applications of the 2D perovskite hybrid was found to be 1.75-2.65 eV.In terms of stability and safety, Bi alkyl ammonium has also been reported as a promising example of a lead-free and eco-friendly hybrid perovskite material for solar cell applications 3,8 .Additionally, novel groups of perovskite materials, such as [(CH 3 ) 2 NH 2 ]Zn(HCOO) 3 , composed of an organic cation and a metal ion, have been discussed [36][37][38][39][40][41][42] .These materials show potential for application in memory manipulation devices and nextgeneration memory storage technology.
In recent times, the potential of perovskites has spurred heightened research into analyzing their structural and mechanical properties.It has been noted that the commonly used CH 3 NH 3 PbI 3 undergoes a phase transition at 329 K, falling within the operational temperature range of solar cells, and exhibits poor photostability.Consequently, ensuring the stability of perovskite solar cell devices emerges as a paramount concern.Moreover, despite their intriguing attributes, materials like perovskites decompose in humid air and pose toxicity risks due to lead.Hence, the imperative lies in developing hybrid perovskites that can be substituted with environmentally friendly alternatives.
This review delves into the potential applications of lead-free organic-inorganic perovskite-type crystals, specifically [NH 3 (CH 2 ) n NH 3 ]MCl 4 (n = 2, 3, 4, 5, and 6; M = Mn, Co, Cu, Zn, and Cd).We seek organic-inorganic hybrid perovskite materials characterized by high or no phase transition temperature, and thermal stability.Through examining the characteristics related to methylene length and the variation of transition metals, our aim is to identify enhanced materials meeting the aforementioned criteria.In this study, single crystals of organic-inorganic hybrid [NH 3 (CH 2 ) n NH 3 ]MCl 4 were grown via the aqueous solution method.We discuss their crystal structure, phase transition temperature (T C ), and thermal decomposition temperature (T d ).The 1 H and 13 C magic angle spinning (MAS) nuclear magnetic resonance (NMR) method plays a crucial role in understanding local dynamics.Determining the spin-lattice relaxation times T 1ρ for 1 H and 13 C is essential for studying dynamical processes.By analyzing the relaxation times of nuclei in different cationic environments, we gain detailed insights into their motion, particularly in the low-to mid-kHz frequency range.Furthermore, we consider the NMR spin-lattice relaxation times T 1ρ , which reflect the energy transfer surrounding 1 H and 13 C atoms.Our results provide a comparative overview of the thermal stability of [NH 3 (CH 2 ) n NH 3 ]MCl 4 , varying methylene length and metal ion as parameters.This review aims to advance the development of lead-free mixtures and the creation of a next-generation predictive model that combines both thermal stability and dynamic metal interactions.

Crystal growth
[NH 3 (CH 2 ) n NH 3 ]MCl 4 (n = 2, 3, 4, 5, and 6; M = Mn, Co, Cu, Zn, and Cd) single crystals were grown using the aqueous solution method.NH 2 (CH 2 ) n NH 2 •2HCl (Sigma-Aldrich) and MCl 2 (Sigma-Aldrich) were dissolved in distilled water.The mixture was stirred and heated, and the resulting solution was filtered.After a few weeks in a constant-temperature bath at 300 K, the single crystals were obtained.

Characterization
At 300 K, the lattice constants were determined via a single-crystal X-ray diffraction (SCXRD) experiment conducted at the Korea Basic Science Institute (KBSI) Seoul Western Center.The experiment utilized a Bruker diffractometer equipped with a graphite-monochromated Mo-Kα target (D8 Venture PHOTON III M14) and a nitrogen cold flow (− 50 °C).Data collection was performed using SMART APEX3 (Bruker 2016) and analyzed with SAINT software (Bruker, 2016).The structure was refined using full-matrix least-squares on F 2 with SHELXTL 114 .Hydrogen atoms were positioned according to the geometric arrangement within the single crystal structure.Additionally, powder X-ray diffraction (PXRD) patterns were obtained at various temperatures using an XRD system with a Mo-Kα radiation source, the same as that used in SCXRD.
Differential scanning calorimetry (DSC) experiments were conducted over the temperature range of 193 to 573 K, employing a heating rate of 10 °C/min under N 2 gas.Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) experiments, utilizing a thermogravimetric analyzer (TA Instrument), were performed within the temperature interval of 300 to 873 K, employing a heating rate of 10 °C/min under N 2 gas as well.

Crystal growth
To obtain single crystals of [NH 3 (CH 2 ) n NH 3 ]MCl 4 (n = 2, 3, 4, 5, and 6; M = Mn, Co, Cu, Zn, and Cd), NH 2 (CH 2 ) n NH 2 •2HCl (Sigma-Aldrich) and MCl 2 (Sigma-Aldrich) were mixed in distilled water according to the molar ratio.The mixture was completely dissolved by stirring and heating to create a saturated solution.These prepared saturated solutions were then placed in a constant-temperature bath at 300 K and slowly evaporated to facilitate the growth of single crystals.Among them, crystals of For n = 2, 3, and 4, the CH 2 close to NH 3 at both ends of the organic chain is denoted as CH 2 -3, the CH 2 in the middle of two CH 2 -3 is designated as CH 2 -2.For n = 5 and 6, the CH 2 close to NH 3 is represented as CH 2 -3, the CH 2 at the center is designated as CH 2 -1, and the CH 2 in the middle of two CH 2 -3 and CH 2 -1 is denoted as CH 2 -2.1a, the Mn atom is coordinated to six Cl atoms, forming an almost regular octahedron, MnCl 6 .Furthermore, the six N-linked hydrogen atoms in one formula unit form N − H•••Cl hydrogen bonds.The structures, space groups, and lattice constants of [NH 3 (CH 3 ) n NH 3 ]MnCl 4 (n = 2, 3, 4, and 5) crystals are summarized in Table 1.It is noteworthy that the crystal structures are monoclinic when n is even and orthorhombic when n is odd.www.nature.com/scientificreports/

Phase transition temperatures
The DSC curves in Fig. 5 depict the thermal behavior of four crystals under a heating rate of 10 °C/min.When n = 2, no peaks were observed 97 .However, for n = 3, two distinct endothermic peaks indicative of phase transition temperatures were observed at 308 K and 338 K.In the case of n = 4, a minor endothermic peak was observed at 378 K. Conversely, for n = 5, an endothermic peak was observed at 298 K 98 .The identified endothermic peaks from the DSC curves were further confirmed as phase transition temperatures through PXRD patterns and polarizing microscopy experiments in response to temperature changes 97,98 .

Thermodynamic properties
To investigate the thermal stabilities, TGA experiments were conducted with a heating rate of 10 °C/min, mirroring the conditions of the DSC experiment.The TGA curves presented in Fig. 6 reveal that crystals with n = 2, 3, 4, and 5 exhibit thermal stability up to approximately 593, 600, 598, and 589 K, respectively.This stability is defined as the thermal decomposition temperature (T d ) corresponding to a 2% weight loss 97,98 .It is noteworthy that these crystals initiate weight loss at higher temperatures.For n = 2, weight losses of 14 and 31% around 628 and 654 K are attributed to the partial thermal decomposition of HCl and 2HCl moieties, respectively.The temperatures at which partial decomposition occurs for HCl and 2HCl in n = 3 and n = 4 are comparable to those observed for n = 2.In the case of n = 5, mass losses of approximately 12 and 24% around 617 and 630 K may be attributed to the loss of HCl and 2HCl, respectively.For methylene lengths of 3, 4, and 5, weight losses of 55% are observed above 700 K, whereas a weight loss of about 48% is seen for n = 2.In summary, these four crystals exhibit relatively high stability at elevated temperatures.

MAS NMR chemical shifts and spin-lattice relaxation times
The     5)°, and Z = 4 73,96 .The structures and space groups of the two crystals resembled monoclinic (P2 1 /c), as detailed in Table 2.For odd values of n, the crystal structure of

Phase transition temperatures
In the DSC thermogram of [NH 3 (CH 2 ) 3 NH 3 ]CoCl 4 with n=3, only one weak endothermic peak was observed at 483 K, as shown in Fig. 10 99 .Meanwhile, in [NH 3 (CH 2 ) 5 NH 3 ]CoCl 4 with n=5, a peak was observed at 494 K. To accurately confirm whether the observed peaks in the two crystals were melting or phase transition temperatures, PXRD results were obtained according to temperature changes. 99These peaks were confirmed to be phase transition temperatures.

Thermodynamic properties
To understand the thermodynamic properties of these crystals, TGA was performed at a heating rate of 10 °C/ min, similar to the procedure used in the DSC experiments.The TGA curves for crystals with n=3 and 5 are presented in Fig. 11.The molecular weights of both crystals decreased as the temperature increased.The amount of residue at higher temperatures was calculated based on the total molecular weight.By considering the number of CH 2 units in the methylene length, the molecular weight loss at approximately 583 and 589 K for n=3 and 5, respectively, marked the onset of partial thermal decomposition, with 2 % weight loss set as T d .For n=3, weight losses of approximately 13 and 26 % around 614 and 633 K could be attributed to thermal decomposition and the partial escape of HCl and 2HCl moieties.The temperatures corresponding to the partial escape of HCl and 2HCl moieties for n=5 were almost similar to those for n = 3, with weight losses of approximately 12 and 24 % near 614 and 632 K due to thermal decomposition and the partial escape of HCl and 2HCl moieties, respectively 99 .The molecular weights of both crystals sharply decreased between 600 and 700 K, with a 50~55% weight loss occurring at approximately 700 K.

MAS NMR chemical shifts and spin-lattice relaxation times
The   The MAS 1 H NMR spectra of NH 3 (CH 2 ) n NH 3 CoCl 4 (n=3 and 5) were recorded, capturing signal intensities at various delay times ranging from 1 μs to 5 ms.The 1 H T 1ρ values, determined from the slopes of the decay rate of proton magnetization, were found to be short, specifically 15.89-19.45μs for n=3 and 16.76-17.22μs for n=5, with respect to changes in temperature.Notably, these values for n=3 and 5 were nearly identical and temperature-independent, as illustrated in Fig. 13.Here, the 1 H T 1ρ values for NH 3 and CH 2 could not be distinguished due to the overlapping 1 H signals of NH 3 and CH 2 .
In contrast, the intensities of the 13 C NMR spectra were measured at various delay times, ranging from 1 μs to 10 ms.The 13 C T 1ρ values for CH 2 -2 and CH 2 -3 were obtained from the slope of their recovery traces.Due to the 13 C baseline being closer to CH 2 -2 for n=3 and CH 2 -3 for n=5, obtaining accurate T 1ρ values was challenging.The 13 C T 1ρ values were determined to be 38-19 μs for CH 2 -3 in the case of n=3 and 25-15 μs for CH 2 -2 in the case of n=5, considering the change in temperature.In both crystals, the 13 C T 1ρ values exhibited a slightly shorter value as the temperature increased, suggesting the activation of molecular motion.Notably, the 1 H and 13 C T 1ρ values of the two crystals with paramagnetic Co 2+ ions were much shorter than the values of crystals without paramagnetic ions.These results align well with the observation that T 1ρ is closely related to paramagnetic ions, being inversely proportional to the square of the magnetic moment 99 .

Crystal structures
The PXRD patterns for [NH 3 (CH 2 ) n NH 3 ]CuCl 4 (n=2, 3, 4, 5 and 6) were obtained at 300 K, as shown in Fig. 14.According to the previously reported case for n=2, the crystal at room temperature exhibits a monoclinic structure with the space group P2 1 /b and Z=2.The unit cell parameters are a=8.109Å, b=7.158Å, c=7.363Å, and    100,103 .However, an endothermic peak at 434 K was observed in the case of n=3, consistent with the structural phase transition 101 .Additionally, an endothermic peak corresponding to the phase transition of the [NH 3 (CH 2 ) 4 NH 3 ]CuCl 4 crystal was detected at 323 K 101 .Finally, upon heating, an endothermic peak was observed for the DSC experiment on the [NH 3 (CH 2 ) 6 NH 3 ]CuCl 4 crystal at 363 K 104 .

MAS NMR chemical shifts and spin-lattice relaxation times
The chemical shift of the 1 H NMR spectrum of [NH 3 (CH 2 ) 3 NH 3 ]CuCl 4 crystals with n=3 was obtained, as illustrated in Fig. 17a.Two peaks in the NMR spectra are designated as NH 3 and CH 2 , and the 1 H signals from NH 3 and CH 2 were observed to slightly overlap.The spinning sidebands for CH 2 are marked with crosses, and those for NH 3 are denoted with open circles.At 300 K, the NMR chemical shift of 1 H for CH 2 was recorded at δ=2.76 ppm, whereas that for NH 3 was observed at δ=11.48 ppm.According to the previous results from our group, the 1 H peak for CH 2 did not significantly change as the temperature increased, while for NH 3 , the chemical shift was temperature-dependent 101 .
The 13 C NMR chemical shifts for CH 2 in the crystals with n=2, 3, 4, 5, and 6 were measured as the temperature increased, and the 13 C chemical shift at 300 K in the case of n=3 is shown in Fig. 17b.At 300 K, two resonance peaks were obtained at chemical shifts of δ=28.78 for CH 2 -2 and δ=124.97ppm for CH 2 -3.The 13 C chemical shifts for CH 2 -2 were different, being far away from NH 3 , while CH 2 -3 was close to NH 3 .
The 1 H NMR spectra were also obtained with several delay times at each temperature, and the intensities of NMR spectra as a function of delay time were represented by a single exponential function.From the slope of the intensity vs. delay times curve, 1 H T 1ρ values were obtained for the CH 2 and NH 3 peaks.For methylene lengths n=2, 3, 4, 5, and 6, 1 H T 1ρ values at 300 K showed similar values in the range of 7-15 ms.The 1 H T 1ρ values for n=2, 3, 4, 5, and 6 are shown in Fig. 18 as a function of inverse temperature.The 1 H T 1ρ values were almost temperature-independent and were in the order of 10 ms.Here, the T 1ρ values were compared according to the cation length from n=2~6, and they exhibited similar trends for different methylene chain lengths, with n=2 exhibiting slightly shorter values than n=3, 4, 5, and 6.
The 13 C T 1ρ values for CH 2 -1, CH 2 -2, and CH 2 -3 in five crystals were obtained as a function of 1000/temperature from the slope of the logarithm of intensity versus the delay time plot.The decay curves for each carbon were represented by a single exponential function.When n=2, 3, 4, 5, and 6, by methylene length, the 13 C T 1ρ at 300 K had values within the range of 1, 39, 32, 60-150, and 37-100 ms, respectively [100][101][102][103][104] .It is interesting to compare the results for 13 C T 1ρ according to the alkyl chain lengths.The 13 C T 1ρ values exhibited similar trends for n=3, 4, 5, and 6, with a very short value for n=2, as shown in Fig. 19.Unlike n=3, 4, 5, and 6, energy transfer was easier for the short alkyl chain length (n=2).

Phase transition temperatures
The DSC curves for [NH 3 (CH 2 ) n NH 3 ]ZnCl 4 crystals, where n ranges from 2 to 6, are presented in Fig. 21.In the case of [NH 3 (CH 2 ) 2 NH 3 ]ZnCl 4 (n=2), the DSC curve did not show any structural phase transition even above the decomposition temperature of 534 K 105 .For [NH 3 (CH 2 ) 3 NH 3 ]ZnCl 4 , only one endothermic peak at 268 K was observed, indicating a phase transition at this temperature 106 .In the case of [NH 3 (CH 2 ) 4 NH 3 ]ZnCl 4 (n=4), two endothermic peaks at 481 and 506 K were identified on the DSC curve, suggesting a phase transition 106 .The DSC result for [NH 3 (CH 2 ) 5 NH 3 ]ZnCl 4 indicated a very strong endothermic peak at 481 K, while two weaker peaks at 256 and 390 K were observed.The phase-transition temperatures obtained from the DSC results were compared with those from SCXRD and PXRD patterns.The small peak at 256 K in the DSC curve was determined to be unrelated to the phase transition.The phase-transition temperature was defined as T C =390 K, and the melting temperature was determined as T m =481 K 108 .In the case of n=6, the DSC thermogram revealed a weak endothermic peak at 408 K and a strong endothermic peak at 473 K. Further confirmation of the phase transition temperature at 408 K and the melting point at 473 K was obtained through PXRD and polarizing experiments, accounting for temperature changes 109 .In the case of n=4, the TGA result remains stable up to approximately 559 K. Weight losses of 12 and 25% due to the loss of HCl and 2HCl moieties occur at temperatures of 604 and 622 K, respectively, with a total weight loss of 95% near 900 K.For [NH 3 (CH 2 ) 5 NH 3 ]ZnCl 4 crystals, molecular weight loss initiates at approximately 583 K, indicating partial thermal decomposition.Two-step decomposition processes are observed: first, a weight loss of 45% occurs at 685 K, and second, a weight loss of 95% occurs at 825 K.The 45% weight loss is attributed to organic decomposition, reaching 90% weight reduction, indicating almost complete decomposition of the organic component, leaving only Zn.Finally, the thermal characteristics of [NH 3 (CH 2 ) 6 NH 3 ]ZnCl 4 crystals were assessed, and the DTA curve exhibited a peak around 473 K, consistent with the melting temperature determined from DSC and optical polarizing microscope analyses.The TGA and DTA curves indicate thermal stability up to about 581 K (T d ).Weight loss rapidly decreases between 600 and 800 K, with approximately 22 % weight loss near 636 K due to the decomposition of 2HCl.Around 800 K, a 90 % weight loss occurs, resulting from the decomposition of NH 2 (CH 2 ) n NH 2 •2HCl in all five compounds.

MAS NMR chemical shifts and spin-lattice relaxation times
The In the case of n=3, MAS 13 C NMR chemical shifts were measured with increasing temperature.Two resonance signals were observed in the MAS 13 C NMR spectra of the compounds.The 13 C chemical shifts for a spinning rate of 10 kHz were obtained, and the chemical shift for 13 C was set as a standard reference for 13 C in TMS.The 1 H MAS NMR spectra were measured with varying delay times, and the plot of spectral intensities vs. delay times was obtained using a single exponential function described by Eq. (1).The 1 H T 1ρ values at 300 K were determined from the overlapping CH 2 and NH 3 peaks by analyzing the slope of the intensities vs. delay times results.The T 1ρ values for the 1 H of NH 3 and CH 2 in [NH 3 (CH 2 ) n NH 3 ]ZnCl 4 (n=2, 3, 4, 5, and 6) were obtained and are presented in Fig. 24 as a function of 1000/temperature.For methylene lengths n=2, 3, 4, 5, and 6, 1 H T 1ρ values at 300 K showed values of 444, 849, 440, 320~410, and 261~510 ms, respectively [105][106][107][108][109] .As the temperature increased, the 1 H T 1ρ values also increased, and the values of T 1ρ above 300 K exhibited variations, as illustrated in Fig. 24.The most notable case was n=2, where T 1ρ gradually increased with rising temperature, reaching a maximum value at 270 K before rapidly decreasing above 300 K. Additionally, T 1ρ reached its minimum value near 380 K and then tended to increase again.This trend between 300 and 430 K indicates the presence of molecular motion.The T 1ρ values influenced by thermal motion are related to the correlation time (τ C ) for molecular motion, according to the Bloembergen-Purcell-Pound theory 116 .Unlike the case of Mn, Co, and Cu containing paramagnetic ions, T 1ρ exhibited very long values in the case of Zn without paramagnetic ions.
The intensities of the 13 C NMR signals were measured by varying the delay times, and the resulting 13 C T 1ρ values for CH 2 -1, CH 2 -2, and CH 2 -3 in the cases of n=2, 3, 4, 5, and 6 were depicted in Fig. 25 as a function of 1000/temperature.These values exhibited similar trends for n=3, 4, and 6.However, the 13 C T 1ρ values for n=2 displayed strong temperature dependence, sharply decreasing above 300 K with increasing temperature.Notably, there was a significant difference between the 13 C T 1ρ values (10 times less) and the 1 H T 1ρ values, indicating that energy transfer for 13 C is more efficient.It is worth mentioning that when n=2, the 1 H and 13 C T 1ρ values exhibited different characteristics compared to n=3, 4, 5, and 6.
Furthermore, the length of the methylene did not distinctly affect the temperature dependence of 1 H and 13 C T 1ρ .Instead, T d , the onset of thermal decomposition, showed temperature changes along the methylene length of the cation.Vol:.( 1234567890 5. Notably, the structures are monoclinic when n is even and orthorhombic when n is odd.

Phase transition temperatures
The DSC curves of [NH 3 (CH 2 ) n NH 3 ]CdCl 4 (n=2, 3, 4, 5, and 6) are presented in Fig. 27.No peak was observed for n=2, while only one endothermic peak at 374 K was observed for n=3.In the case of n=4, two endothermic peaks were observed at 341 K and 366 K 110 .For the [NH 3 (CH 2 ) 5 NH 3 ]CdCl 4 crystal, two endothermic peaks were observed at 336 K and 418 K 112 .The enthalpy for the phase transition is 3.17 kJ/mol at 336 K and 0.55 kJ/mol at 418 K, respectively.Finally, in the case of [NH 3 (CH 2 ) 6 NH 3 ]CdCl 4 , two endothermic peaks with enthalpies of 3.27 and 0.93 kJ/mol were observed at 337 K and 472 K, respectively 113 .These two peaks correspond to the phase transition temperatures.www.nature.com/scientificreports/

Thermodynamic properties
The TGA curves presented in Fig. 28 reveal that the [NH 3 (CH 2 ) n NH 3 ]CdCl 4 crystals with n=2, 3, 4, 5, and 6 are nearly stable up to approximately 587, 592, 595, 595, and 587 K, respectively [110][111][112][113] , marking the onset of partial thermal decomposition corresponding to the number n of CH 2 groups in the carbon chain.These compounds undergo breakdown near 650 K, experiencing a loss in molecular weight as the temperature rises.The remaining amount as solid residues can be calculated based on the molecular weights.In the case of n=2, a loss of 12 and 23% of its weight at temperatures of about 622 and 804 K was attributed to the partial decomposition of HCl and 2HCl, respectively.Similarly, weight losses of 11 and 22% occurred at temperatures of 613 and 623 K in the case of n=3.For [NH 3 (CH 2 ) 4 NH 3 ]CdCl 4 with n=4, at temperatures of 612 and 623 K, 11 and 21% of their weight were lost, respectively.In the case of n=5, the 10 and 20% weight losses at temperatures of about 617 and 626 K were attributed to the partial thermal decomposition of HCl and 2HCl, respectively.Furthermore, weight loss at approximately 800 and 900 K was observed to be 46 and 87%, respectively.Finally, in the case of n=6, the weight loss rapidly decreased between 600 and 800 K, with a weight loss of about 20% occurring near 625 K due to the decomposition of 2HCl.Notably, when Cd is included, there is a significant difference in weight loss at high temperatures.Specifically, a loss of 25% in the case of n=2 occurs around 800 K, while n=3, 4, and 5 show a loss of 45%, and n=6 exhibits a loss of 65 %.

MAS NMR chemical shifts and spin-lattice relaxation times
The   The 1 H NMR spectra were measured with various delay times at each temperature for five crystals, and the slopes of the intensities vs. delay times followed a single exponential function.From the slope of the logarithm of intensities vs. delay times, the 1 H T 1ρ values were obtained for NH 3 and CH 2 .These values are shown in Fig. 30 for five crystals as a function of 1000/temperature.In the case of n=2, 3, 4, and 5, the 1 H T 1ρ values increase rapidly as the temperature rises, and those of n=2 and 3 rapidly reduce at high temperatures.The 1 H T 1ρ values  of n=6 exhibited a slight dependence on temperature.It can be seen that 1 H T 1ρ values according to the n values are different at high temperatures [110][111][112][113] .
The intensities of the 13 C NMR spectrum showed changes due to various delay times.The 13 C T 1ρ values, obtained from the slope of their recovery traces, were determined for CH 2 -1, CH 2 -2, and CH 2 -3.Their results for five crystals are shown as a function of 1000/temperature in Fig. 31.In the case of n=2, 13 C T 1ρ decreased slightly with a rise in temperature and decreased rapidly at high temperatures.For n=3, 4, 5, and 6, the 13 C T 1ρ values decreased slightly with an increase in temperature, and increased again.

Conclusion
In the pursuit of applications for an improved lead-free organic-inorganic perovskite-type solar cell, our research focused on identifying conditions conducive to organic-inorganic hybrid perovskite materials with high or no phase transition temperature, and high thermal stability.Consequently, we sought improved organic-inorganic hybrid perovskite materials by exploring the characteristics related to methylene length and the variation of various transition metals.
Single crystals of the organic-inorganic hybrid [NH 3 (CH 2 ) n NH 3 ]MCl 4 (n=2, 3, 4, 5, and 6; M=Mn, Co, Cu, Zn, and Cd) were grown using the aqueous solution method, and their crystal structures, phase transition temperatures (T C ), and thermal decomposition temperatures (T d ) were thoroughly examined.Additionally, our investigation delved into the impact of the even-odd number in the methylene length and various transition metals, holding potential implications for future applications.
From a structural standpoint, compounds involving Mn, Cu, and Cd, consisting of octahedral (MCl 6 ) 2− units, exhibited a monoclinic structure when n was even and an orthorhombic structure when n was odd.In contrast, for Co and Zn compounds with tetrahedral (MCl 4 ) 2− units, the crystal structure displayed an orthorhombic or triclinic arrangement when n was even and a monoclinic structure when n was odd.
Surprisingly, the phase transition temperatures did not exhibit a clear trend based on different transition metals (M=Mn, Co, Cu, Zn, and Cd), nor did they show a consistent pattern with varying methylene lengths of n=3, 4, 5, and 6.Remarkably, in the case of n=2, possessing the shortest methylene length, no phase transition temperature was observed.Notably, when the transition metal was Co, a relatively high phase transition temperature was recorded, as detailed in Table 6.
The results of comparative analysis and explanations for research on the thermal stabilities of [NH 3 (CH 2 ) n NH 3 ]MCl 4 , where the methylene length varies as a parameter, are depicted in Fig. 32.In the case of transition metals Mn and Cd with an octahedral MCl 6 2-structure, the thermal decomposition temperature (T d ) remains almost constant with respect to the methylene length, whereas for Co and Zn with a tetrahedral MCl  and rapid decrease.The changes in thermal decomposition temperature (T d ) according to the transition metals can be explained by electronic configuration considerations.In the cases of Mn 2+ and Co 2+ , where the 3d electron of the M shell is not filled and the valence electron is 4s 2 , as well as in the cases of Zn 2+ and Cd 2+ , where the 3d electrons of the M shell and the 4d electrons of the N shell are filled, and the valence electrons are 4s 2 and 5s 2 , respectively, T d remains almost constant or increases as the methylene length increases.However, in the case of Cu 2+ , where the 3d electrons of the M shell are filled and the valence electrons are 4s 1 , T d tends to decrease as the methylene length increases.
The NMR chemical shifts were found to be associated with the local field around the location of the resonance nucleus in the single crystals.Upon including metal ions (Mn, Co, Cu, Zn, and Cd) in [NH 3 (CH 2 ) n NH 3 ]MCl 4 (n=2, 3, 4, 5, and 6), the 1 H and 13 C chemical shifts exhibited the following trends: All 1 H chemical shifts for the twenty-one compounds appeared at almost identical positions.However, the 13 C chemical shifts displayed marked differences between the cases of paramagnetic ions (Mn, Co, and Cu) and the cases of Zn and Cd, which do not contain paramagnetic ions.Notably, 1 H and 13 C chemical shifts based on the methylene length n did not reveal any unusual patterns or trends.
The experiment results presented in Tables 7 and 8 highlight notable differences in 1 H T 1ρ values based on the presence or absence of paramagnetic ions.When paramagnetic ions such as Mn, Co, and Cu are included, 1 H T 1ρ exhibits very short values, whereas when Zn and Cd, which lack paramagnetic ions, are present, 1 H T 1ρ values become considerably longer.Interestingly, 13 C T 1ρ shows similar values regardless of the presence of paramagnetic ions.This suggests that 1 H has a significant impact on T 1ρ , while 13 C has a minimal effect due to   its distance from the paramagnetic ion.Examining T 1ρ values across different methylene lengths (n) reveals no significant differences or unusual patterns.
Considering the overall trends of 1 H and 13 C T 1ρ , n=3, 4, 5, and 6 exhibit almost similar trends as temperatures rise.In contrast, for n=2, a similar trend to n=3, 4, 5, and 6 is observed at low temperatures, but it undergoes a rapid shortening phenomenon at high temperatures.A longer T 1ρ implies increased difficulty in energy transfer from the nuclear spin to the surrounding environment.Consequently, the physicochemical properties of organic-inorganic hybrid perovskite [NH 3 (CH 2 ) n NH 3 ]MCl 4 , influenced by various transition metal ions and the methylene length of the cation, hold potential applications as materials with lead-free, and high thermal stability attributes.
1 H NMR spectra of [NH 3 (CH 2 ) n NH 3 ]MnCl 4 (n = 2, 3, 4, and 5) crystals were measured using NMR spectroscopy at various temperatures.Figure 7a displays the 1 H NMR chemical shifts for [NH 3 (CH 2 ) 5 NH 3 ]MnCl 4 with n = 5.The resonance lines observed at lower temperatures are asymmetric due to the overlap of signals representing NH 3 and CH 2 .The line widths denoted by A and B on the left and right sides of the half-maximum in Fig. 7a are not equal.Above 300 K, the NH 3 and CH 2 signals are resolved, with chemical shifts of 9.29 and 2.89 ppm, respectively.Spinning sidebands are marked with + and o to represent 1 H in NH 3 and CH 2 , respectively.
1 H NMR spectra of [NH 3 (CH 2 ) 3 NH 3 ]CoCl 4 crystals recorded by MAS NMR experiment at 300 K were represented in Fig. 12a.The chemical shift of the resonance peak was observed at 6.40 ppm as a single resonance line.The spinning sideband is marked with open circles.The observed resonance line was symmetric, corresponding to the overlapping lines of NH 3 and CH 2 ; the line widths denoted by A and B on the left and right sides of the halfmaximum are identical.Thus, the chemical shifts of NH 3 and CH 2 did not separate and completely overlapped.
1 H NMR chemical shifts of [NH 3 (CH 2 ) n NH 3 ]ZnCl 4 crystals (n=2, 3, 4, 5, and 6) were recorded using MAS NMR spectroscopy.In the case of [NH 3 (CH 2 ) 3 NH 3 ]ZnCl 4 with n=3, only one peak in the NMR spectra was observed, resulting from the overlap of NH 3 and CH 2 signals.The observed resonance signal was asymmetric, as illustrated in Fig. 23a.The line widths, represented as symbols A and B at the half-maximum value, differ from those at 3.54 and 6.11 ppm, respectively 106 .This asymmetry is attributed to the overlapping lines of the two 1 H signals for CH 2 and NH 3 in the [NH 3 (CH 2 ) 3 NH 3 ] cations.At 300 K, the 1 H NMR chemical shift was observed at δ=6.74 ppm.Spinning sidebands were marked with open circles and crosses.
MAS 1 H NMR spectra of [NH 3 (CH 2 ) 3 NH 3 ]CdCl 4 crystal with n=3 were recorded at 300 K, as shown in Fig. 29a.These resonance signals appeared asymmetric due to the overlapping of the 1 H resonance lines of NH 3 and CH 2 in the [NH 3 (CH 2 ) 3 NH 3 ] cation.The spinning sidebands for NH 3 and CH 2 are denoted with open circles and crosses.At 300 K, the 1 H chemical shift for CH 2 was δ=3.23 ppm, whereas that for NH 3 was δ=7.67 ppm 111 .The 13 C MAS NMR chemical shifts for CH 2 in [NH 3 (CH 2 ) 3 NH 3 ]CdCl 4 were recorded at different temperatures.At 300 K, the two resonance signals appeared at δ=25.04 and 39.07 ppm for CH 2 -2 and CH 2 -3, respectively, as shown in Fig. 29b.The 13 C chemical shifts for CH 2 were different for CH 2 -2, far away from NH 3 , and CH 2 -3, close to NH 3 .The line width of CH 2 -3 is wider than that of CH 2 -2.